There are several prior art devices that make electrical connection to the pins of a dual in-line package (DIP) of the sort commonly used to encapsulate integrated circuits. Among these is the "clothes pin" DIP clip. This type of device has a pair of plastic legs or sides pivotally hinged together and forced apart at the top by a spring under compression. This forces the bottoms of the sides together. The sides have a width (parallel to the pivotal hinge axis) generally equal to the length of the DIP upon which the clip is to be used upon. Molded in the bottom portion of the sides are narrow teeth and a recess that serve to grip the DIP both by friction and mechanical interference. Imbedded in the sides are a number of rods; one for each pin of the DIP. At the lower end the facing sides of the rods are exposed, and when placed upon the DIP the pressure of spring urges the rods against the pins of the DIP. At the upper end the rods are extended above the tops of the sides, and serve as posts onto which various conventional probe means are to be attached. Basically, the DIP clip is simply an upward mechanical extension of the pins of the DIP as they go down into the printed circuit board. And while that extension is generally mechanical secure, it is subject to some limitations and some disadvantages.
The foremost disadvantage of the DIP clip is that a given clip fits DIP's of only a very narrow range of widths. This is because the lower edges of the clip will spread apart only so far when the top edges are squeezed together. The same DIP clip that fits on a DIP 0.25 inches in width will not also fit onto a DIP of 0.9 inches in width. Another disadvantage arises from the fact that the DIP clip requires the additional connection of the actual electrical probe to the tops of the rods. There is always the worrisome possibility that the probe will, during its application or removal, cause a short between two adjacent rods. This difficulty can be mitigated by turning off the power to the device under test, but that remedy is inconvenient and time consuming at best, and may cause loss of the very conditions to be investigated. Yet another disadvantage of the DIP clip is the susceptibility to noise and cross-talk of the long unshielded rods in the DIP clip. Still another disadvantage of the DIP clip is that it comes in fixed lengths, say, fourteen pins (total, seven per side), twenty pins, etc. And while that is generally good, there are times when one needs to look at the signals of just one or two pins on an eight pin package and the only DIP clips in the drawer or junk box are too long (too many pins) to fit on the part because of interference with adjacent parts.
One needn't use a DIP clip to attach probes to a DIP; there are probes means to attach directly to the pins of the DIP itself. There are a variety of prior art hook-type probe tips, of both the single hook compression type and the opposing double hook scissors type. They all have some common annoying features. They are prone to shorting adjacent pins of the DIP. They can be hard to get hooked, and even harder to get unhooked. And frequently, they are unshielded over a significant portion of their length.
It would be desirable if there were a way to attach probe tips to the pins of a DIP without the worry of shorting adjacent pins. It would also be desirable if such a probe clip were directly usable on DIP's of widely different widths. Lastly, it would be nice if the thing were easy to get on and get off.
A clip-on probe for a DIP will have these desirable features if it is constructed in accordance with the one described herein, a brief summary of which is as follows. A probe assembly for use on a DIP includes a probe body onto which is attached a detachable gripping portion. The detachable gripping portion includes an extensible claw with a plurality of fingers for interlaced positioning with pins on the DIP. The extensible claw is formed at one end of a pair of flexible struts attached at a distal end to a slidable cylindrical body. The slidable cylindrical body is slidably mounted upon the outside of a hollow core having a bore running through a longitudinal axis, and is slidable back and forth along the direction of that axis. At one end of the cylindrical body and on either side of an aperture where the bore opens are a pair of prongs extending generally further along the direction of the axis, although somewhat below it. Somewhat above the bore, across from the prongs, are a pair of curved guides, in which the flexible struts travel. The curved guides subtend an angle of about fifty degrees of arc. As the flexible struts are extended through the curved guides the extended portions are held parallel (by the user) to the top surface of the DIP to be connected to. As a result of the curvature of the curved guides the probe body and its longitudinal axis are inclined up away from the plane of the printed circuit board by about fifty degrees, the better to provide clearance for the hand and fingers when attaching and removing the probe. A compression spring urges the slidable cylindrical body along the core in a direction that retracts the extensible claw toward the prongs. If not mounted upon a DIP the claw interferes with the curved guides to retain the cylindrical member upon the core. If mounted upon a DIP the DIP itself itself will resist the retraction, while the fingers in the claw and the prongs interlock on opposite sides with pins of the DIP. The fingers of the claw and the prongs are inclined inwards to a point beneath the bottom of the DIP, and thus exert a pinching action to retain the probe assembly on the DIP. The pair of prongs straddle one pin of the DIP. The geometry is so arranged that the bore opens out onto that pin. The detachable portion slides over the probe body; in the process a shielded contact extends through the bore. An unshielded portion of the contact extends through the bore between the prongs. It is this extended portion of the shielded contact that makes an electrical connection to the pin of interest on the DIP. The detachable portion is held onto the shield by the action of some interfering lugs on the bore and a recessed ring at a corresponding location in the shield. It is held with sufficient strength to resist the expanding push of the compression spring; the exposed contact is thus urged against the pin of the DIP by the full force attempting to retract the claw toward the prongs. A finger grip on the core and a thumb-pad on the cylindrical body allow the thumb and forefinger to extend the claw while positioning the probe upon a DIP. The tight fit that holds the detachable portion onto the probe body is also sufficient to allow the claw to be extended by a pushing motion on the thumb pad that is not countered by a pulling motion on the finger grip.
The probe automatically adjusts to the width of the DIP to which it is being attached. The length of the flexible struts and the freedom provided for the cylindrical body to travel over the core are principally what limit the width of the DIP, by determining how far the claw can be extended. Regardless of how far the claw is extended, it does so in approximately a straight line motion away from the prongs, and the compression spring supplies the force to obtain the same degree of interfering grip of the fingers and prongs upon the DIP.
Short circuits are not possible when applying the probe to a DIP. First, the diameter of the contact, extending from the bore and between the prongs, is insufficient to bridge the gap between two adjacent pins. The only way it could bridge the gap is if it were held at a considerable angle, so that its length could span the gap between the pins. But that can't happen either, because not enough of the contact extends out of the bore, and because of the interfering action of the prongs. The only way the exposed end of the contact can touch a pin is if the prongs are straddling a pin, and that precludes the conditions that would allow the contact from simultaneously being in the proximity of another pin.
Since the contact is shielded almost all the way the probe has excellent noise immunity. Finally, the probe is easily attached and removed with natural motion with the thumb and forefinger of one hand.